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Tectonic processes and hazards (Earth structure and plate tectonic theory,…
Tectonic processes and hazards
Earth structure and plate tectonic theory
Earth structure
Compositional/chemical layer
Crust
Thin layer. Continental-70km thick. Oceanic-9km thick. Granite+basalt. May reach 400 degrees.
Mantle
Thick layer. Rich in iron, magnesium. Between 700-2,900km deep. Upto 870 degrees.
Core
Iron+Nickel. Centre+7,000 degrees. Hotter than surface of sun.
Mechanical/physical layers.
Lithosphere
Crust+upper mantle. Thin layer. Solid and forms the lithospheric plates which float on the asthenosphere
Asthenosphere
Fluid/plastic part of mantle. Same materials as upper layer of mantle. Pressure+heat increased making it
rheid
(solid that flows).
Mesosphere
Lower part of mantle-pressure here really starts to increase making this layer solid
Outer/inner core
Outer=liquid. Inner=solid.
Crusts
Continental crust
30-70km thick. Over 1,500 million- 3.8 billion years old. Average density=2.7g/cm3. Granite.
Oceanic crust
5-9km thick. Under 200 million years old. Average density=3.0g/cm3. Basalt
Theory of continental drift-Alfred Wegner 1912.
Is the theory that continents shift overtime. However Wegner was unable to suggest how/provide evidence.
Evidence to support this theory.
Tectonic fit
Fragments of an old belt of mountains are found on widely separated continents. Pieces found in Greenland, Canada, Ireland, UK, Scandinavia.
Glacial deposits
Are found in Antarctica, Africa, SA, India and Australia. When continents reassembled, we can see likely size and extent of the sheet.
Geological fit
Some areas of SA+Africa have rocks of same age and composition. Meaning rocks must of formed under the same conditions in the same place.
Fossil evidence
Many examples of fossils found on separate continents and no where else, suggesting continents were once joined.
Jigsaw fit
Similarity of continental coastlines. Continents fit together suggesting they were once part of a larger landmass. A supercontinent called
Pangea
Sea floor spreading-Harry Hess 1962
Suggested, oceans growing from their centres molten materials oozing up from Earths mantle to create new ocean floor at mid ocean ridges(underwater mountain ranges).
This new oceanic crust pushes the tectonic plates apart in a process known as sea floor spreading. As oceans become wider, continents drifted apart.
Palaeomagnetism
Palaeo=old
Study of past changes in earths magnetic field. During underwater eruptions as magma cools, strongly magnetic minerals in the magma align. with earths magnetic field
Pattern found of magnetic stripes on either side of mid-oceanic ridges. As earths magnetic field reverses periodically. Formed when magnetic pole was in north, new lava aligned to north.
Plate boundaries
What is the global distribution of volcanoes and earthquakes
Volcanic and earthquake activity is mostly found at plate boundaries, e.g. Pacific Ring of Fire , Indonesia, Iceland, Italy and Hawaii for volcanoes; Chile, Japan, Western USA for earthquakes.
Constructive/destructive boundaries. Volcanoes are also found intra-plate, e.g. hotspots (Hawaii).
Earthquakes occur at all plate boundaries and can occur on smaller, ancient transform faults
What causes plate tectonics to move
2.Seaf-floor spreading
Mid-ocean ridges formed when hot magma escapes from mantle and hardens making new oceanic crust. This new crust pushes tectonic plates apart.
3.Subduction
As new crust is created In one place its destroyed by subduction. As two plates collide, denser one slides under into mantle, where it melts in subduction zone. Creating mechanism of slab-pull.
1.Mantle Convection
Heat produced by decay of radioactive elements in earth core heat lower mantle. As its heated it rises, creating convection currents, that move in cellular motion in asthenosphere causing plates above to move.
4. Slab-pull
As the plate subducts, its weight pulls it further/faster into mantle. The pull of gravity on a plate that's being subducted.
Types of plate boundary
Divergent
Where plates move apart also known as
constructive
Transform
Plates slide past eachother also known as
conservative
Convergent
Where 2 plates collide also known as
destructive
Plate margins
Convergent Margin – oceanic and continental plate
Accretionary wedge
- as sediments on the downgoing plate are scraped off and accumulated
Fold mountains / volcanoes
- as the two plates collide, forcing the continental plate to be folded and slowly pushed up (referred to as an orogeny)
Deep ocean trench
- as oceanic plate subducts beneath the continental plate
Benioff zone
– an area of seismicity due to friction created by the plate being thrust downwards
E.g Andes mountain range. S.America
Hydration melting
– water brought down into the mantle during subduction lowers the melting temperature of the rocks in the mantle, creating gas-rich magma which can lead to explosive volcanic eruptions
Convergent Margin – oceanic and oceanic plate
Deep ocean trench
Benioff zone
Hydration melting
Island arc
– magma rises to create underwater volcanoes. Over millions of years these growing volcanoes rise above sea level to form a chain of separate island volcanoes
E.g Japan
Convergent Margin – continental and continental plate
Fold mountains
When two continental plates meet, both plates have the same density, and are less dense than the mantle beneath them, so neither plate is subducted.
E.g Himalayas
Divergent margin
E.g Mid-Atlantic ridges
Decompression melting
- as hot magma rises towards the gap between the plates, pressure lowers enabling the rock to melt. This creates gas-poor magma which tends to lead to gentle eruptions
Transform faults
– as seafloor spreading takes place, it happens more quickly in some places than others
When plates move apart on continents, the crust stretches and breaks into sets of parallel cracks (faults). The land between these faults then collapses, forming steep-sided valleys called
rift valleys
.
Transform Margin
As plates slide past each other, it causes a major break in the crust between them known as a fault
E.g San Andreas fault, USA
Hazard definitions and models
Hazard definitions
A natural hazard
is a natural event that has the potential to threaten both life and property (Whittow)
Without people it is just a
natural event
not a hazard, it needs the interaction of people to make it a hazard.
A disaster
is a major natural hazard that causes significant social, environmental and economic damage.
When a disaster becomes a mega-disaster
According to UN...
OR Over 200,000 become homeless
OR A 5% drop in GDP
Over 2,000 deaths
OR Dependant on foreign aid for more than a year
The risk equation
RISK=(Hazard x Vulnerability)/Capacity to cope
Risk
is the exposure of people to a hazardous event. More specifically it is the probability of a hazard occurring that leads to the loss of lives and/or livelihoods.
Vulnerability
is to be susceptible to injury or damage. The ability to anticipate, cope with, resist and recover from a hazard. A vulnerable community is susceptible to injury or damage.
Hazard profiles
Definitions
Duration
Length of time that the hazard exists
Areal extent
Size of area covered by the hazrad
Speed of onset
Time difference between start and peak of the event
Spatial predictability
Distribution of the hazard: where it occurs in particular locations e.g plate boundaries or hotspots.
Magnitude
The size of the event: for example the size of an earthquake on the ritcher scale
Frequency
How often an event of a certain size occurs. Also called recurrence interval.
Volcanoes
Stratovolcano/composite volcano
Differences to shield volcano
Infrequent
High viscosity
Ash+lava
Andesitic lava
Narrow, high, steep sided
Sticky
Explosive with pyroclastic flows, volcanic bombs+tephra.
Destructive margins/convergent
Shield volcano
Differences to Stratovolcano
Frequent
Basaltic lava
Wide, low height
Runny
Effusive, mostly lava, no pyroclastic flows
Low viscosity
Constructive+hotspots
Types of lava
Andesitic
Violent, moderately explosive, traps gases
Composite cone volcanoes, subduction zones, E.g Mt. Pinatubo Philippines
Violent, ticker, High silica+Gas content.
800-1,000 degrees
Rhyolitic
Supervolcano or composite cone volcanoes, E.g Yellowstone USA.
Very violent
Coolest, very violent, thicker, high silica+gas content.
Sticky, high viscosity, traps gases
650-800 degrees
Basaltic
Hottest type, low viscosity, gentle, low silica+Gas content. Kilauea Hawaii.
Ocean hotspot. Mid ocean ridges, shield volcanoes.
1,000-1,200 degrees.
Supervolcano
Larger than normal, eruption 1,000xs greater
Rising magma cant escape, large bulge on surface
Cracks appear in surface Gas+Ash erupt from chamber
Chamber collapses forming a depression called a
Caldera
(Normal volcano forms a cone shape)
E.g
Yellowstone
last erupted 2.1million years ago. 60 mile long, 30 mile wide.
Different types of seismic waves
P-Waves(body wave)
Travels fastest
Moves through solid and liquid (body wave)
Arrive first
Compressional waves (push and pull action).
S-Waves(body wave)
Arrive second.
Only moves through solid rock (Body wave).
Travel slower than P waves (60% speed of P-waves – 4 km/s).
Up and down movement.